By Nadim Ladki

BEIRUT, June 7 (Reuters) - France and Saudi Arabia are promoting efforts to widen Lebanon's cabinet to end a protracted crisis pitting the Western-backed government against Hezbollah and its allies, political sources and diplomats said on Thursday.

They said the new mediation effort aimed at defusing tension ahead of a presidential election later in the year.

Lebanon was plunged into its worst political crisis since the 1975-1990 civil war when opposition ministers quit the cabinet of Prime Minister Fouad Siniora last November after the ruling majority refused opposition demands for veto power.

The majority accused the opposition, made up mainly of Christian and pro-Syrian Shi'ite Muslim factions, of trying to block passage of an international tribunal to try suspects in the 2005 killing of ex-premier Rafik al-Hariri.

Pro-government leaders say Syria was behind Hariri's assassination. Damascus denies any hand in it.

The opposition took to the streets from December 1 to press its demands, setting up a tent camp in central Beirut that has blocked business in the heart of the capital ever since.

Siniora, backed by the United States, France and Saudi Arabia, refused to cave in.

The crisis paralysed parliament, rendering it unable to approve the statutes for the U.N.-backed Hariri tribunal, and damaged an economy still trying to recover from a devastating July-August war between Israel and Hezbollah.

Security deteriorated, with a fresh wave of bombings and battles between the army and al Qaeda-inspired militants holed up in a Palestinian refugee camp in the north.

But after the U.N. Security Council approved formation of the tribunal last week, Saad al-Hariri, parliamentary majority leader and son of the slain ex-premier, offered an olive branch to the opposition, galvanising efforts to resolve the crisis.

Political sources said French diplomats suggested to the anti-Syrian majority that it accept the opposition demand for veto power in the cabinet now that the court had been passed.

Paris wanted a smooth run-up to the election of a new president between late September and late November, they said.

In Lebanon, parliament elects the president, who is always a Maronite Christian under the country's complex power-sharing formula. If the assembly fails to choose a president, a power vacuum or creation of two rival administrations might ensue.

The French move encouraged Saudi Ambassador Abdul-Aziz Khojah to resume an earlier mediation effort in coordination with the Iranians, who are Hezbollah's main backers.

"We are close to a formula to resolve the government crisis," a senior opposition source said. "We are putting the final touches but we have to be cautious as we have been down this road before only for things to unravel quickly."

The sources said the deal would expand Siniora's original 24-member government to 30, with the opposition allocated more than 11 seats, the number necessary to block key decisions.

Iran's ambassador, Mohammad Reza Shibani, confirmed the revived efforts to resolve the crisis after meeting Parliament Speaker Nabih Berri, who is also a main opposition leader.

"There are joint French, Saudi and Iranian efforts and, more importantly, domestic Lebanese political efforts," Shibani said.

(Additional reporting by Laila Bassam)

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